Add spec load pass to check for undefined build args

[adamperlin]

What this PR does / why we need it:
Currently, if a build arg is not specified in the args section of a Dalec spec file, its value is set to "" when the spec is loaded. This is confusing behavior, as a user could easily forget to declare a build-arg or mistype a build-arg reference, leading to an empty string substitution which could be hard to track down.

This PR adds an additional pass upon loading the spec which tokenizes all the build args first, and checks to ensure that each referenced arg does appear in the args section of the spec, raising an error if not.

Which issue(s) this PR fixes (optional, using fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when the PR gets merged):
Fixes #274

Special notes for your reviewer:

[cpuguy83]

Did you happen to look into using https://pkg.go.dev/github.com/moby/buildkit/frontend/dockerfile/shell?utm_source=godoc#Lex.ProcessWordWithMatches instead?
I believe that should give us the list of vars which weren't in our env list.

[adamperlin]

Ah no, I was not aware that function existed. I will see if I can reimplement using that instead

[adamperlin]

It looks like the version of this function you referenced exists in a newer version of buildkit (0.14.0) than the one we import in dalec (0.13.1). Are we ok to bump the buildkit dependency?

[cpuguy83]

Oh nice, yes we can/should bump it.

[cpuguy83]

MInor readability nit, but otherwise LGTM